Atomic spectroscopy finds extensive application in areas of atomic emission, atomic absorption and atomic fluorescence. In particular, spectrometers of the echelle variety, in which an echelle grating is used to disperse the radiation of interest, have proven to have certain advantageous characteristics, related to resolution and spectrometer design, when compared to spectrometers using conventional, high ruling density diffraction gratings. In an echelle spectrometer, greater resolution of the spectral lines derives from the fact that resolution increases as the diffraction order increases. Thus, the use of orders 20-90, for example, will yield greater resolution than the use of orders 1, 2 and 3, for example. Also, the physical size of the spectrometer is reduced by use of an echelle diffraction grating.
When using an echelle grating, the various diffraction orders normally lie essentially atop one another. A prism is commonly used to separate the orders after diffraction by the grating, and a two-dimensional pattern is formed at the focal plane in the spectrometer. The detector then comprises a two-dimensional surface containing sensing elements to detect the spectral lines.
Various types of optically-detecting semiconducting material devices have been used to collect and detect the photon signals present on focal planes of various configuration in analytical spectrometers. Linear and area arrays of photodiodes, charge-coupled devices, charge injection devices, and plasma coupled devices have been used to retrieve optical signals from conventional diffraction grating spectrometers as well as echelle grating spectrometers.
As one example of a charge-coupled device that has been employed to detect the two-dimensional diffraction patterns in echelle spectrometers, an x-y grid of pixels having 1000 rows and 1000 columns of pixels, or a total of 1,000,000 pixels, may be placed at the focal plane to detect the spectral lines. This arrangement has the advantage of being able to detect virtually all signals within the spectrometer, including background noise. Disadvantages of this form of detector include mismatch between the information sought and the detector array, loss of portions of the spectrum, and unnecessarily long read-out time. The two-dimensional diffraction pattern for continuous wavelength coverage at a given diffraction angle will form a "keystone" pattern, rather than a rectangle. The use of one or more x-y grid detectors means a mismatch of shapes between the detector and the diffraction pattern. Tradeoffs exist between underfilling/overfilling of the detector and loss of portions of the diffraction spectrum. In addition, because every pixel is read in the x-y grid, read-out time is high, even though most of the pixels will have no useful information.
Another version of solid-state detector is reflected in U.S. Pat. No. 4,820,048, to Barnard. Barnard, in effect, eliminates most of the pixels in the x-y grid, leaving small sets of linear CCD arrays positioned so as to receive only selected spectral lines and nearby background radiation. This drastic reduction in the number of pixels reduces the read-out time for the detector, but any radiation not falling at the selected line positions will not be sensed, decreasing the flexibility of this detector as compared to the x-y grid.
Therefore, an object of the present invention is to provide more efficient and effective means to match analytical spectral signals in a focal plane of an analytical spectrometer with semiconductor solid-state optical detector devices.
Another object of the present invention is to provide complete pixel coverage of each echelle diffraction order over the free spectral range at that diffraction angle while not unduly increasing the read-out time required.
Another object is to provide a detector having arrays of sensing elements positioned exactly along and on the locations of spectral signals on a focal plane of an echelle grating spectrometer.
Yet another object of the present invention is to provide a solid-state detector having skewed lines of pixels so that there is exact registry between the pixels and the configuration of the diffraction orders so there is continuous spectral coverage of the entire echelle grating focal plane which is produced by a particularly designed echelle grating spectrometer.
Still another object is to provide a detector having skewed lines of pixels, wherein each skewed line covers a specific diffraction order and has a number of pixels which is chosen to match the number of resolution elements available in that order over the free spectral range.
Another object is to provide a detector having skewed lines of pixels, wherein each skewed line covers a specific diffraction order and has a length which is chosen to match the length of that order of the free spectral range.
Another object is to provide a detector having skewed lines of pixels, wherein each skewed line covers a specific diffraction order and has pixels of sizes chosen to match the sizes of the resolution elements available in that order.
Still another object is to provide a detector having arrays of pixels along the locations of the diffraction orders such that the lines track the angles and the curvature of the diffraction orders as those angles and curvature change.
Another object of the invention is to provide a detector having pixels arranged along the locations of the diffraction orders so there is a continuous acceptance of spectral information across the orders.